TN (Twisted Nematic) type liquid crystal display devices and STN (Super Twisted Nematic) type liquid crystal display devices, which are now widely used, are inferior in view angle characteristics because electrochemical characteristics vary with a view angle, thus causing a large problem in applications such as TV in which view angle characteristics are considered to be important. As a method of attaining a wider view angle, a VA (Vertically aligned) system (VA mode) (see Non-Patent Document 1), an IPS (In-Plane Switching) system (IPS mode) (see Non-Patent Document 2) and an ECB (electrically controlled birefringence) system (ECB mode) have been proposed up to the present. As a liquid crystal material used in VA-LCD (Vertically Aligned Liquid Crystal Display), a liquid crystal material having a negative dielectric constant anisotropy is required, unlike TN type and STN type liquid crystal display devices (see Patent Document 1) and the following characteristics are required: 1. high response speed, 2. high nematic phase-isotropic liquid phase transition temperature (TN-I), and 3. low nematic phase-isotropic liquid phase transition temperature. That is, there is required a display which has higher-speed responsibility to a desired driving voltage and is more stable within a wider temperature range. It is also considered to be important that electrostatic capacity of TFT element causes less variation on driving of a liquid crystal. In case of the same driving voltage, it is required to develop a liquid crystal which exhibits a negative dielectric constant anisotropy having a smaller absolute value or more steep electrochemical characteristics. Furthermore, there is required a liquid crystal composition having a higher nematic phase-isotropic liquid phase transition temperature (TN-I) and a lower viscosity, capable of driving at a low voltage.
To solve the above problems, there have been employed the following liquid crystal materials having a 2,3-difluorophenyl group (see Patent Documents 2 to 7):
wherein R and R′ represent an alkyl group or an alkoxy group.
The compound represented by the general formula (a) shows a high absolute value of the negative dielectric constant anisotropy, however, a liquid crystal composition using a large amount of this compound has such a problem as to decrease the nematic phase-isotropic liquid phase transition temperature (TN-I). The liquid crystal composition using the compound represented by the general formula (b) has comparatively high nematic phase-isotropic liquid phase transition temperature (TN-I), but has such a problem that the absolute value of the negative dielectric constant anisotropy is not large.
The liquid crystal compound having a 1,7,8-trifluoronaphthalene-2,6-diyl group has already been known (see Patent Document 8) and has already been applied to the nematic liquid crystal composition having a negative dielectric constant anisotropy (see Patent Documents 9 and 10). In the above publications, however, there was not such a specific description about the kind of the compound used to prepare a composition in order to achieve the above object in a liquid crystal composition using a compound having a 1,7,8-trifluoronaphthalene-2,6-diyl group. Said compound itself is characterized by a large absolute value of the negative dielectric constant anisotropy and a high nematic phase-isotropic liquid phase transition temperature (TN-I), but had such a problem that a liquid crystal display device using said compound shows a low response speed because of comparatively high viscosity.
As described above, it is not easy to provide a liquid crystal composition which has a large absolute value of the negative dielectric constant anisotropy, a high nematic phase-isotropic liquid phase transition temperature (TN-I) and a low viscosity. Thus, it was required to develop a liquid crystal composition having excellent characteristics.    (Patent Document 1)    Japanese Patent Application, First Publication No. Hei 11-242225 (page 1)    (Patent Document 2)    Japanese Patent Application, First Publication No. Hei 8-104869 (page 2)    (Patent Document 3)    Japanese Patent Application, First Publication No. Hei 10-176167 (page 2)    (Patent Document 4)    Japanese Patent Application, First Publication No. Hei 11-140447 (page 2)    (Patent Document 5)    Japanese Patent Application, First Publication No. 2001-192657 (page 2)    (Patent Document 6)    Japanese Patent Application, First Publication No. 2001-316669 (page 2)    (Patent Document 7)    Japanese Patent Application, First Publication No. 2002-201474 (page 2)    (Patent Document 8)    German Patent    German Patent Application No. 19522195A (page 2)    (Patent Document 9)    Japanese Patent Application, First Publication No. 2001-40354 (page 2)    (Patent Document 10)    Japanese Patent Application, First Publication No. 2002-69449 (page 2)    (Non-Patent Document 1)    Ohmuro et al., Society for Information Display (SID) Digest, Society for Information Display (SID), 1997, p 845-848    (Non-Patent Document 2)    Ohta et al., Asia Display Digest, Society for Information Display (SID), 1995, p 707-710